1. Field of the Invention
The present invention relates to a manufacturing method of an optical waveguide for a touch panel.
2. Description of the Related Art
Optical waveguides, which are typically incorporated in optical waveguide devices, optical integrated circuits, optical wiring boards and the like, are widely used in the fields of optical communications, optical information processing and other general optics. Lately, the optical waveguides are produced by using a photosensitive resin such as a UV-curable resin. For example, an under-cladding layer is formed on a substrate, and then a photosensitive resin layer is formed on the under-cladding layer. In turn, the resulting photosensitive resin layer is exposed to light via a photo mask having a predetermined pattern, and developed by removing an unexposed portion of the photosensitive resin layer, whereby cores are formed having the predetermined pattern. Then, an over-cladding layer is formed over the cores. Thus, an optical waveguide is produced (see, for example, JP-A-2005-165138).
In the optical waveguide production method disclosed in JP-A-2005-165138, a material applying step, an exposing step, a developing step and a drying step are performed for the formation of each of the cladding layers (the under-cladding layer and the over-cladding layer) and the cores. However, where a process sequence including the applying step, the developing step, the exposing step and the drying step is performed for the formation of each of the under-cladding layer, the over-cladding layer and the cores, the number of process steps is increased, thereby increasing production costs. Particularly, the exposing step and the developing step each include many sub-steps, requiring improvement.
A conceivable method for reduction in the number of process steps is, for example, to employ a die-forming method for the formation of the over-cladding layer. As shown in FIG. 9(a), an under-cladding layer 2 and cores 3 are first formed in this order on a substrate 1. Then, a molding die 20 formed with a recess 21 having a die surface 21a conformable to the surface geometry of the over-cladding layer 4 (see FIG. 9(b)) is prepared, and a thermosetting resin sheet 40 is prepared as a material for the over-cladding layer 4. In turn, the thermosetting resin sheet 40 is positioned above the under-cladding layer 2 and the cores 3. Then, as shown in FIG. 9(b), the thermosetting resin sheet 40 is pressed on the under-cladding layer 2 by the molding die 20 and, in this state, heated and cured, whereby the over-cladding layer 4 is formed. Thereafter, the molding die is removed. This method obviates the applying step, the exposing step and the developing step for the formation of the over-cladding layer 4. This reduces the number of process steps and the costs required for the formation of the over-cladding layer 4.
In the method in which the thermosetting resin sheet 40 is pressed by the molding die 20, however, the thermosetting resin sheet 40 is sandwiched between the under-cladding layer 2 and the molding die 20, so that burrs 41 are liable to occur on edges of the formed over-cladding layer 4. This additionally requires the step of removing the burrs 41, so that the reduction in the number of process steps and the costs is unsatisfactory. Where the optical waveguide produced in the aforementioned manner is used for a touch panel, a portion of the over-cladding layer 4 associated with distal ends of the cores 3 has a lens shape (curved surface) 4a as shown in FIG. 9(b). Therefore, the burrs 41 are present on an edge of the lens-shaped portion 4a, and are likely to prevent convergence of light and the like.
To cope with this problem, the applicant of the present invention has proposed the following optical waveguide production method, and filed a patent application (JP-Application-2007-123830) related to this method. In this production method, when an optical waveguide (see FIG. 10(c)) including an under-cladding layer 2, cores 3 of a predetermined pattern and an over-cladding layer 4 is produced on a surface of a substrate 1, a special molding die is used for formation of the over-cladding layer 4. That is, the under-cladding layer 2 and the cores 3 of the predetermined pattern are formed on the surface of the substrate 1 in the same manner as described above by performing the applying step, the exposing step, the developing step and the drying step (see FIG. 10(a)). In turn, as shown in FIG. 10(a), a molding die 10 of a light-transmissive material formed with a recess having a die surface conformable to the surface geometry of the over-cladding layer 4 and through-holes 12 communicating with the recess is prepared, and an opening of the recess of the molding die 10 is brought into intimate contact with a surface of the under-cladding layer 2. In this state, a liquid resin as a material for the over-cladding layer 4 is injected into a cavity defined between the die surface of the recess and the surface of the under-cladding layer 2 through the through-holes 12, and then exposed to radiation such as ultraviolet radiation through the molding die as shown in FIG. 10(b). Thus, the injected material is cured. Then, the molding die 10 is removed. Thus, the over-cladding layer 4 is formed, in which the cores 3 of the predetermined pattern are buried as shown in FIG. 10(c) (see JP-Application-2007-123830).
In the production method disclosed in JP-Application-2007-123830, however, the over-cladding layer material should be injected into the molding die 10 through the through-holes 12, and a certain time period is required for the curing of the material from the injection of the material. Therefore, this method is unsatisfactory in productivity, thereby requiring improvement. When the over-cladding layer material is injected into the cavity through the through-holes 12, air is liable to be trapped in the cavity, resulting in occurrence of voids. Therefore, the optical waveguide, if used for a touch panel, requires improvement in product reliability.
In view of the foregoing, it is an object of the present invention to provide a touch panel optical waveguide production method that ensures improved productivity while suppressing occurrence of voids.